Multiple-neck filter



Dec. s, 1953 W. VAN B. ROBERTS MULTIPLE-NECK FILTER Original Filed March 30, 1949 a a z5'y n INVENTOR )fm me :MN5/9055975 BY i ATTORNEY Patented Dec. 8, 1953 MULTIPLE-NE CK FILTER Walter van B. Roberts, Princeton, N. J., assignor to Radio-Corporation of America, a corporation of Delaware Griginal application 4March 30, 1949, Serial No. 84,372. Divided and this application .uly 8, 1950', Serial No. 172,746

(Cl. S33-71) 8-Claims. i.

This application is a division of the copending application, Serial No, 84,372, filed March 30, 1949, now Patent No. 2,647,948, issued on August 4, 1953.

This invention relates to electromechanical filters. More particularly, it relates to ban'dpass lters (BPFs) 0f the mechanically-vibrating type.

As described in said application, several advantages are obtained through the use of mechanical resonators in filters for low radio frequencies. Such resonators are small, inexpensive and have very high Qs as compared to those of electrical circuits. According to said application, BPFs having extremely narrow pass-bands have been obtained by the use of mechanical resonator structures. Such structural arrangements generally consist of at least two like tuned mechanical resonator elements, aligned but spaced apart, with other elements positioned between and coupling together adjacent resonant elements. Such coupling elements have mechanical impedances different from that of the resonant elements or tanks. In effect, such coupling elements function to loosely couple together the tanks.

In a mechanical filter, the characteristic or mechanical impedance of each portion (tank and coupling portions) is the product of its lcross sectional area and the intrinsic impedance of the material of which it is made, the `latter quantity` .11,

being in turn the product of its density and the velocity of propagation of longitudinal waves along that portion. If rall thev iilter is madeA of the same material, the ratio n of the mechanical impedance of the coupling *portionA to that off'the tank portions is simply the ratio of cross vsections, assuming that' the sections are small enough so thatv the compressional wave velocity is 'the' same in all portions. Filterswhich have a 'value of p` small compared toY unity are known as neck-type or neck-coupled lt'ers, and the present invention relates generally to filters of this type. In such filters, the coupling elements are of smaller transverse dimensions than the tank elements.

As disclosed in said application, for a BPF p must be ofthe order of the` desired fractional bandwidth. The fractional bandwidth is deiined as the ratio of the width of thel lter passband in cycles per second to the mid frequency of the passba-nd in vcycles per second. If a onep'er'cent band vis required, this means that-ar necks-*type filter turned out of round stockmust have a neck diameter only about one-tenth that of the tanks. It may therefore be `seen that narrown'ess ofthe 2 band -is vlimited by ilimsiness of' thestructure; Matters are even worse if the lter is vcutout of iiat strip material, because in this ycase the neck width must be p times the tank width.

In a neck-type nlter, the. tank elements: 'are connected' together by a neck so thin asto'ract at some frequencies like a weak spring. The smaller the cross section' of thespring, the 'weaker is this spring and the morek loosely the tank lelements are coupled together. Also, the smaller the cross section of the spring or neck coupler, the smaller is the ratio p andthe smaller is "the fractional band-width. This is `true sinne, fas previously stated, p is of the order ofthe Trace tional bandwidth. Therefore, in general, 'the more loosely the tanks `or resonators are coupled, the narrower is the Width of the filter passband;

An object of this invention is to devise-.a .mechanical BPF having a Very'narrow passband.

Another objectis'to` disclose a simple and satisfactory method for obtaininga very'narrow pass'- band in an electromechanical BPF.

A further object is to disclose a method 'for obtaining very loose coupling between the resonant elements of a mechanical filter.

A still furtherl object is to devise an arrangement permitting extremely loose couplingv between tanks, in a vneck-type filter, without requiring 'unduly small necks.

An additional object is to provide a neck-type mechanicalBPF having a very narrow passband; without requiring unduly small necks.

The foregoing and other objects of the invention will be best understood from the following description of some examples thereof, reference` beinghad to the accompanying drawing, where` of a -lter according to this invention; and- Fig. 2 is a partly diagrammatic representationv of another lter according to this invention.

The objects .of this invention are accomplished, briefly, in the following manner: Sudsidiary lters, operating in. an attenuating band,.are used as coupling elements between the tanks in a neck-coupled mechanical filter. Each of these subsidiary filters or coupling elements consists of at least two spaced neck coupling elements with intervening elements of larger cross section. Thus, multiple-neck coupling elements are utilized in the filter. The end neck elements of the subsidiary lters or low-pass filter (LPF) sections are connected to the tanks or 'resonators of the main filter. Thusthe entire arrangement in effect consists of a composite filter, comprising Fig. l isa `somewhat schematic representation I main filter resonators coupled by means of subsidiary filters.

Fig. 1 is a representation of a filter according to this invention. The lter is positioned between two permanent magnets I and 2 which establish magnetic flux longitudinally therethrough. The filter is metallic and may be either fiat (being punched out of strip or sheet stock) or may be formed as figures of revolution, having circular cross section throughout. In Fig. l, multipleneck coupling elements are utilized. This figure shows a single-section neck-type filter coupled by twin elements. This filter consists of a pair of aligned half-wave resonant elements or tanks 3 and 4 coupled by twin quarter-wave necks 5 and 6 separated by a quarter-wave slug l'. The necks 5 and 8 are relatively thin compared to the tanks 3 and 4. The slug 1 is relatively thick compared to necks 5 and 6, so that its acts somewhat as a heavy mass. Both ends of each resonator 3 and 4 are moti-unal loops at the frequency of operation.

Element 3 is the drive element or tank, while element 4 is the take-off tank. The filter 3-1 is mechanically driven by means of a coil 8 coupled to element 3, and take-off to an electrical circuit is by means of a coil S coupled to element 4. Far magnetostrictive drive and take-od, the material used for the drive and takemff tanks must, of course, be magnetostrictive. For this purpose. nickel-plated aluminum may be used for the end tanks. This nickel plating makes possible magnetostrictive driving of, and take-ofi from, the tanks, in accordance with the principles disclosed in the copending Burns application, Serial No.

84,373, led March 30, 1949, now Patent No. 2,619,604, issued November 25, 1952. Nickel has good magnetostrictive activity. The elements 5, 6 and 1 may be of this same magnetostrictive material or of suitable other metallic material, for

example brass or nickel.

Driving coil 8 is coupled to the center portion of drive tank 3. Coil 8 is in eiTect wound around resonant element 3. Said coil is tuned by a condenser connected in parallel therewith and is connected to the output circuit of a driving amplier tube l0. Tube l0 is supplied from a source of oscillatory energy, which source is of low radio frequency, for example, anywhere in the range of fty to lve hundred kilocycles, the upper limit being set chiefly by the decreasing dimensions of the parts. Coil B actingon tank 3 produces alternating longitudinal magnetization.

Take-off coil 9 is coupled to the center portion of take-off tank 4. Coil 9 is in effect wound around this resonant element. Said coil is tuned by a condenser connected in parallel therewith and is connected to the input circuit of an amplier tube l l. Tube l I may be termed a signal utilization means.

The polarizing magnets l and 2 apply a longitudinal magnetic field to tanks 3 and 4. Although two separate magnets are illustrated, it will be appreciated that a single horseshoe magnet could be used instead, if desired.

The combination of the constant longitudinal magnetization (produced by l and 2) and the alternating longitudinal magnetization producedby driving coil 8 results in longitudinal vibrations of resonator 3. This is due to magnetostrictive action.

The composite filter 3--1 is metallic and may be formed from a single piece of material. It can be either at, in which case it is punched out of strip or sheet stock, or round, in which case it is machined out of suitable round stock.

The coupling element 5 1 of Fig. 1 extends between tanks 3 and 4. This coupling element is of the multiple-neck type, being specifically of the twin-neck type. The coupling element itself consists of twin quarter-wave necks 5 and 6 separated by a quarter-wave slug 1. Elements 5, 1 and 6 are joined together end-to-end in that order. The remaining end of neck 5 is joined to tank 3 and the remaining end of neck 6 is joined to tank 4.

The multiple-neck coupling element 5-1 permits extremely loose coupling between tanks 3 and 4 without requiring an excessive ratio of the cross Sections of tanks 3 and 4 to those of the necks. A physical picture of the operation of the coupling system 5-1 may be had by considering element 5 adjacent to tank 3 as the actual coupling element while the other two, 1 and 6, act as two quarter-wave transformers in tandem which transform the impedance of the other tank 4 to a value still further out of line with that of the coupling element 5. The impedance transformation, more specically, is equal to the square of the ratio of the individual impedances. This results in a narrower band than that provided by a lter having a single-neck coupling, for the same ratio of impedances of the elements. For practical purposes, it is usually sufficient to gure that the lters passband is narrower by the factor p as compared to the corresponding filter with a single neck coupling element.

Although the slug 1 is shown as having approximately the saine transverse dimension as the tanks 3 and 4, this does not necessarily need to be the case. Slug 1 may have either a larger of smaller transverse dimension than do the tanks 3 and 4, as long as the mechanical impedance of said slug has the proper relation to that of necks 5 and 6. Making the slug 'l of the same transverse dimension or of the same impedance as tanks 3 and 4, facilitates analysis and is a convenient construction.

Another way of looking at the multiple coupler 5-1 is to consider it as a subsidiary filtering means or LPF operating above cut off. The coupler 5, 6, 1 can be considered a single section LPF. It therefore attenuates, in a nondissipative manner, vibrations passing through in either direction. Hence, it operates in an attenuating band at the frequency of operation of the filter section, which is determined by the frequency of the voltage applied to coil 8. This LPF connects or serves as the coupling between adjacent resonators 3 and 4. The subsidiary filter 5 1 attenuates as a result of mechanical impedance differences between certain of its elements and the remaining elements of the composite filter. From this point of view, it is evident that other forms of subsidiary filters, operating in an attenuating band, could be employed as loose coupling means between tanks of the composite lter 3 1. The composite filter can be said to consist of resonators 3 and 4 coupled by the subsidiary filter 5-1.

tudinal motion of the magnetostrictively-active.

acca-,21:1

5 -mairerialY of. tank f4, ran-V a'lternating: vnltagas 4inducedinrccil Sand fed toutiiizaticn meansclrlr. v .Another factor pmay be: obtained. by adding another pair ci' .quarter-Wave elements; te the coupling element .of Fig. Lftofform'atniple-neck coupling. In. other words, the. band. :may ;be narrowed 'still :further by fadding ririoreqzzairs of large and small quarter-wave elements tothe coupler; This will make the `coupling. :between ithe tanks still. looser, Yhecausleitv prtcwidesI .more attenuation in the lter-type' coupling :sections Thus, .by usinga sulicient number .off elements in the coupler it .is passible to obtain aanam'ew a band fas desired, requiring. to: :be impractically'small;

Fig. 2 illustrates :a 'filter ofzthe type just'. referred to. figure shows a .twoese'ction (multiple-section) neck-'type iilter with tripleneck (multiple-neck) coupling; 'The dniye end tank 'I.=2 and the atake-.oir end.- tank. I3 vare :each half-wave resonators. Here, as in Fig. i1,;al1 of theelements are aligned vand. lconnected in endto-end relationship. End tank I2 is joined to a large-central resonator I4 by a series of' three quarter-wave neck elements I5, I6 and I'I and two quarter-wave slug elements I 8 and I9, the large and small (slug and neck) elements alternating. between tank I2 and resonator I4. YOne end of end'neck I5'o`the cnupling element iii-ISS is joined to tank I2. One end` of the opposite end neck I'Il of coupler Iii-i9, is joined to-centra'lV resonator I'4. Resonator lI4 is a one-Wailea length resonator.

The three necks I5, I6 and I'I, in conjunction with slugs I8 and I9, form a triple-neck coupling between tank I2 and resonator I4. Multiple-neck coupler I5-I9 provides a subsidiary iilter, operating in an attenuating band, to serve as loose coupling means between resonators I2 and I4. The triple-neck coupler of Fig. 2 provides a narrower lter passband than does the twin-neck coupler of Fig. l. The coupling element I 5-I9 can be considered a two-section LPF.

Similarly, end tank I3 is joined to resonator I4 by a series of three quarter-wave neck elements 20, 2I and 22 and two quarter-wave slug elements 23 and 24, the slug and neck elements alternating between tank I3 and resonator I4. One end of end neck 2i] of coupling element 20-24 is joined to tank I3. One end of the opposite end neck 22 of coupler 20-24 is joined to centralresonator I4. The three necks 29, 2| and 22, in conjunction with slugs 23 and 24, form a triple-neck coupling between tank I3 and resonator i4. The multiple-neck coupler 20--24 likewise provides a subsidiary lter, operating in an attenuating band, to serve as loose coupling means between resonators I3 and I4. The coupier 20-24 can be considered a two-section LPF.

As in Fig. l, end tanks I2 and I3 are magnetized longitudinally by permanent magnets I and 2 and are located in drive and take-off coils B and 9, respectively. Longitudinal vibrations occur due to magnetostrictive action.

The Fig. 2 arrangement is a two-section composite lter in that each half of the structure, that is, one end tank, half the central resonator, and the quarter-wave elements between them, is a single-section filter. The two sections of this filter are placed end-to-end. In this connection, it is desired to be pointed out that, if desired, two or more single-section composite lters of the Fig. 1 type could be joined endtf1-end. to` provide: a rmxlti'fsectien, twinenecke.

coupled .iiltergV Such .icining .together` endeteend of the plurality nix-sections couidbefeiected in accordance;y with. the; arrangement .illustrated inFigz..

Fig-.2: shows', tor example,.:a. two-sectionniter made of` sheetinickel' thathas, inthe .design-illus.- trated', Vone'jpassband from 94:7. to :9.57 klocynles andv another from 2id 110.2795;y kilocyclea. In this vpartimilar, designs., theendr; tanica-I2? armi d were each one inch long.. The; central element I4 was two inches long, Velements iig-214; eachbeing 1/2-.inch lpng. vhe ratio of widths 4oathe wide Yand narrow elements wasa 32.1'.. 'The .onerialllength of: this ii'lter.,k excluding permanent magnets, wastnine inches.

Also, if desired, the couplingt element. of the single-:section: vFig; Llter fcouldi bei modiedin acnordancev with.v the:[teaching:oztlig.` 2 to pncilideV a. :single-sectipn,- triple-.neckeoupledilter. l

Although. the .iilten` fof'. Fig; 2l isi illustrated: as. having onlyT twozy sections. it; is desired tof .be paintedv out that-,the concept: of Fig; 2 mayA extended: byv thexj oini-ng .on of' :additional secticns in ende-to-enrl relationship; to` fjzirmlde` aY filter ofgmore than twoz'sectiofns.

What I claim to) bemy invention is asgziollowsa;

l. A .mechanical lter. section,v comprising.r sa; pair* oi spaced :resonant: elements and meansf therebetween,l said meansr'consistingfof fa'. coupling elem-ent hailing` a. plurality of; portions of .different.mechanicalimpedances the length; of each of said.. plurality 'of portions .being sub-f. stantially al quarter-Wavelength atrthe frequency of operation of said lter section.

2. A mechanical filter section, comprising a pair of spaced resonant elements and coupling means therebetween, said means consisting of a multipartite coupling element having two portions of a certain mechanical impedance and at least one portion positioned therebetween of a different mechanical impedance, said two portions being joined one to each of said resonant elements, the length of each portion of said multipartite coupling element being substantially a quarter-wavelength at the frequency of operation of said lter section.

3. A mechanical filter section, comprising a pair of spaced resonant elements and multipartite coupling means therebetween, said means consisting of two portions of a certain mechanical impedance and one portion, of a higher mechanical impedance, positioned between said two portions and joining the same together, said two portions being joined at their free ends one to each of said resonant elements, the length of each portion of said multipartite coupling means being substantially a quarter-wavelength at the frequency of operation of said filter section.

4. A mechanical filter section, comprising a pair of spaced resonant elements and coupling means therebetween, said means consisting of a multiple coupling element which includes two spaced segments having a certain mechanical impedance,

two spaced segments of a higher mechanical impedance, and one other segment of a mechanical impedance less than said higher impedance, said one segment being positioned between and joining together said two segments of higher impedance, said one segment and said two segments of higher impedance being positioned between and joining together said two segments of certain impedance, and said two segments of certain impedance being joined at their free ends one to each of said resonant elements, the length of each segment 7 of said multiple coupling element being substantially a quarter-wavelength at the frequency of operation of said lter section.

5. A multisection mechanical filter, comprising a plurality of identical sections connected in endto-end relationship, each of said sections consisting of a pair of spaced resonant elements joined by coupling means, each coupling means consisting of a multiportion coupling element having two portions of a certain mechanical impedance and at least one portion positioned therebetween of a different mechanical impedance, said two portions being joined one to each of said resonant elements, the length of each portion of said coupling element being substantially a quarter-'wavelength at the frequency of operation of said filter.

6. A multisection mechanical filter, comprising a plurality of identical sections connected in endto-end relationship, each of said sections consisting of a pair of spaced resonant elements joined by coupling means, each coupling means consisting of a coupling element having two portions of a certain mechanical impedance and at least one portion positioned therebetween of a higher mechanical impedance, said two portions being joined one to each of said resonant elements, the length of each portion of said coupling element being substantially a quarter-wavelength at the frequency of operation of said filter.

7. A mechanical filter providing a band-pass characteristic, comprising a pair of spaced resonant elements tuned to a frequency within said pass band, and coupling means therebetween,

said coupling means comprising, in the order named, a neck element of lesser dimension than the resonant elements, a slug element of larger dimension than said neck element, and a second neck element to constitute at least one low pass mechanical filter section which operates in an attenuation band at the frequency of operation of said filter.

8. A mechanical filter providing a band-pass characteristic comprising, in the order named, a rst resonator element, a first coupling neck, a first coupling slug, a second coupling neck, a second coupling slug, a third coupling neck, and a second resonator element, said first and second resonator elements being tuned to resonate at a frequency within the pass band of the filter, said coupling necks having smaller cross-sectional dimension than said slugs, said coupling necks and slugs constituting a subsidiary filter having an attenuation band at the band pass frequency of the filter.

WALTER VAN B. ROBERTS.

References Cited in the le of this patent UNITED STATES PATENTS Number Name Date 1,933,306 Berry et a1 Oct. 31, 1933 2,055,996 Braden Sept. 29, 1936 2,231,404 Blackman et al Feb. 11, 1941 2,342,813 Mason Feb. 29, 1944 2,342,869 Kinsley Feb. 29, 1944 2,501,488 Adler Mar. 21, 1950 

